1. Field of the Invention
This invention relates to an exhaust gas purifying catalyst.
2. Description of Related Art
Three-way types of catalytic convertors are devices installed in an engine exhaust line to significantly lower emission levels of carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). This in turn causes carbon monoxide (CO) and hydrocarbons (HC) to change into water vapor (H.sub.2 O) and carbon dioxide (CO.sub.2) by means of an oxidizing process. On the other hand, it changes nitrogen oxides (NOx) into nitrogen (N.sub.2) and oxygen (O.sub.2) by means of a deoxidizing or contact cracking process. A most popular three-way type catalytic converter uses platinum (Pt), palladium (Pd) and rhodium (Rh) as a catalyst material which is carried by G-aluminum. Such a three-way catalytic converter causes an increase in the rate of a chemical reaction or exhaust gas burning for air-to-fuel (A/F) ratios near a theoretical air-to-fuel (A/F) ratio of 14.7.
Emissions, especially nitrogen oxides (NOx), can pose a health problem for human bodies and ecological systems if uncontrolled. For this reason, engines must be equipped with devices to lower emission levels of nitrogen oxides (NOx) as low as possible. For vehicle engines, it is practical to equip a catalytic converter and an emission control by placing same in the exhaust system between the exhaust manifold and muffler. On the other hand, in order to meet exhaust gas regulations and fuel consumption regulations, what are called "lean-burn engines" are already put to practical use. Because such a lean-burn engine burns a lean fuel mixture which, in turn, is generally at a high air-to-fuel ratio and, consequently, discharges exhaust at a high concentration of oxygen (O.sub.2), although the three-way catalytic converter causes the oxidization of carbon monoxide (CO) and hydrocarbons (HC), it can not deoxidize nitrogen oxides (NOx), so that it is difficult to reduce emission levels of nitrogen oxides (NOx).
From this technical background, one of the promising and attractive catalysts is a zeolite type catalyst, which bears a transition metal carried by means of ion-exchange, to catalytically crack or change nitrogen oxides into nitrogen (N.sub.2) and oxygen (O.sub.2) either directly or by the aid of catalytic agents, such as carbon monoxide (CO) and hydrocarbons (HC), even if the exhaust gas is at a high concentration of oxygen. Efforts are focused on various improvements of such an ion-exchanged transition metal carrying zeolite catalyst so that it can reduce nitrogen oxides (NOx) in a high rate and to cause an increase in the rate of chemical reaction. For example, a zeolite catalyst can be improved in the rate of nitrogen oxides (NOx) purification and in durability by carrying therein copper (Cu), at least one of alkaline-earth metals and at least one of rear-earth metals. Such a zeolite catalyst is known from, for example, Japanese Unexamined Patent Publication No. 3 (1991) - 202157.
In general, Cu ion-exchanged zeolite type catalysts can effectively reduce emission levels of nitrogen oxides (NOx), and exhibit, on an experimental basis, a rate of nitrogen oxides (NOx) reduction higher than 90%. However, when such a Cu ion-exchanged zeolite type catalyst is utilized as a practical catalytic converter in lean-burn engine mounted vehicles, it causes a great decrease in the rate of nitrogen oxides (NOx) reduction. This, on one hand, is considered to result from the difference in conditions between a model gas used in experiments and an exhaust gas actually discharged from such a lean-burn engine. Nitrogen oxides (NOx) have a great tendency to be in the form of nitrogen dioxide (NO.sub.2) when the exhaust gas is at high concentrations of oxygen and at temperatures lower than 200.degree. C. The Cu ion-exchanged zeolite type catalyst is inactive on nitrogen oxides (NOx) at temperatures lower than 200.degree. C. However, temperatures at which a nitrogen oxides (NOx) reduction activity is actualized are within a temperature range of 350.degree. to 450.degree. C. Also, the Cu ion-exchanged zeolite type catalyst is evaluated to be lower in the rate of nitrogen oxides (NOx) reduction for the engine exhaust gas at high oxygen concentrations. Further, because, in particular, the exhaust gas, discharged from an engine being operated under lower loads, is lower in gas temperature at the entrance of a catalytic converter, it is also essential for the Cu ion-exchanged zeolite type catalyst to be active in chemical reaction over a wide range of temperatures including lower temperature.